The present invention pertains to a tantalum sputtering target capable of enhancing the evenness (uniformity) of a film and improving the quality of sputter deposition, and a method for manufacturing a tantalum sputtering target having the foregoing properties by performing forging, annealing, rolling processing, heat treatment and so on to a tantalum ingot or billet having been subject to melting and casting.
In recent years, the sputtering method for forming a film from materials such as metal or ceramics has been used in numerous fields such as electronics, corrosion resistant materials and ornaments, catalysts, as well as in the manufacture of cutting/grinding materials and abrasion resistant materials.
Although the sputtering method itself is a well-known method in the foregoing fields, recently, particularly in the electronics field, a tantalum sputtering target suitable for forming films of complex shapes and forming circuits is in demand.
Generally, this tantalum target is manufactured by forging and annealing (heat treatment) an ingot or billet formed by performing electron beam melting and casting to a tantalum material, and thereafter performing rolling and finishing processing (mechanical processing, polishing, etc.) thereto.
In this kind of manufacturing procedure, the forging performed to the ingot or billet will destroy the cast structure, disperse or eliminate the pores and segregations, and, by further annealing this, recrystallization will occur, and the precision and strength of the structure can be improved to a certain degree.
Generally, the ingot or billet subject to melting and casting has a crystal grain size of roughly 50 mm. By performing forging and recrystallization annealing to the ingot or billet, the casting structure will be destroyed, and, for the most part, even and fine (100 μm or less) crystal grains can be obtained, but there is a problem in that these coarse crystals will remain even in the final product.
Generally, upon performing sputtering, the finer and more uniform the crystals of the target, the more even the deposition, and a film having stable characteristics can be obtained.
Therefore, the existence of irregular crystal grains that are generated during forging, rolling or the annealing to be performed thereafter will change the sputtering rate, and there is a problem in that evenness (uniformity) of the film will be affected, and the quality of sputter deposition will deteriorate.
Further, if a forged product with strain remaining therein is used as is, the quality will deteriorate, and this must be avoided at all costs.
Meanwhile, when sputtering a tantalum target with a magnetron sputtering device, only a certain area along the magnetic line will in particular become eroded (generally, the erosion will progress in a donut shape), and this will gradually become steeper together with the progress of erosion up to the end of sputtering.
At portions where erosion progresses in particular, the surface area of the target will increase, and the difference in the surface area in comparison to the other areas will become significant. This difference in surface area will become the difference in the sputter rate, and the film tends to be formed thick at the substrate (wafer) portion positioned opposite to the portion with an increased surface area where sputtering is focused, and, contrarily, the film tends to be formed thinly at portions where sputtering is not as focused.
This not only generates a problem in that an uneven film is formed in a single wafer, this will also generate a problem in that the film thickness will vary from the start to end in the plurality of wafers to be sputtered. In other words, this will lead to the deterioration in the uniformity of sputter deposition.
As a method of improving the uniformity of such sputter deposition, it has been generally proposed that the structure should be made uniform as possible, and in particular to align the crystal orientation across the entire thickness direction of the target. Nevertheless, by merely aligning the crystal orientation, there is a problem in that the deterioration of the sputtering film uniformity resulting from the variation in the foregoing surface area could not be resolved.
As a conventional manufacturing method of a tantalum sputtering target or high purity tantalum, a manufacturing method of high purity tantalum has been disclosed (e.g., Patent Document 1 below) comprising the steps of melting and highly purifying a tantalum sputtering target containing 500 ppm or less of metallic impurities and K2TaF7, reacting the highly purified K2TaF7 with a reducer and obtaining tantalum powder, and reacting this tantalum powder with iodine in a container.
Further, a 99.95 wt % tantalum sputtering target having an equiaxed structure of (100) and a maximum grain size of 50 microns or less manufactured by rolling and forging has also been disclosed (e.g., Patent Document 2 below).
Moreover, a high purity tantalum target having a fine structure and capable of uniform sputtering, in particular, a high purity tantalum target of a fine structure in which the average crystal grain size is 100 μm or less and (111)<uvw> is preferentially orienting evenly toward the thickness direction of the target has also been disclosed (e.g., Patent Document 3 below).                Patent Document 1: PC (WO) 2002-516918        Patent Document 2: PC (WO) 2002-518593        Patent Document 3: U.S. Pat. No. 6,331,233        